1.Field of the Invention
This invention relates to communication systems. More specifically, this invention relates to high frequency amplifiers.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Included among conventional microwave amplification systems are microwave tubes and microwave solid state devices. Microwave tubes are currently employed in large-signal amplification systems and as microwave sources. However, high-voltage requirements, comparatively large physical dimensions, and the occasional need for stabilization circuitry have limited the utility of microwave tubes in certain applications. Microwave solid state amplification devices include microwave transistors as well as negative resistance diodes. Transferred electron devices (TEDs), also referred as Gunn effect devices, constitute one class of commonly used negative resistance diodes, and are some of the most widely used.
The Gunn-effect relates to periodic current fluctuations within uniform semiconductors when an applied DC voltage exceeds a certain predetermined threshold value. A DC voltage may be applied to a Gunn-effect diode to induce an oscillating current which can be utilized as a signal source and circulator-coupled networks have been used in conjunction with Gunn-effect diodes to realize amplifier circuits.
Unfortunately, these amplifier networks have been found to be complex and expensive to manufacture. Further, Gunn-effect devices suffer from low efficiency when used as amplifiers at frequencies above 10 to 15 GHz. Thus, microwave field effect transistors (FETs) are gaining in acceptance as the preferred means of small-signal microwave amplification.
Microwave FETs have inherent advantages relative to earlier microwave bipolar transistors. The efficiency and maximum frequency of amplification of microwave FETs exceed those of comparable bipolar devices. In addition, microwave FETs generally exhibit a low noise figure.
Hence, the combination of low noise and high operation frequencies have made microwave FETs one preferred means of amplification in low-noise receivers. Similarly, the low-noise and high efficiency of microwave FETs have been significant factors in the acceptance of FETs as alternatives for negative resistance diodes such as TEDs in low power applications.
Despite the advantages of microwave FETs over microwave tubes and negative resistance diodes, microwave FETs are subject to the same gain-bandwidth constraint applicable to other semiconductor devices. Moreover, the inherent parasitic feedback capacitance present in FETs also limits high-frequency power gain and can contribute to undesired oscillation. Further, as gate widths are reduced in the fabrication of higher frequency FETs, the current handling capability is correspondingly decreased.
These constraints have limited the power gain of microwave and millimeter wave FETs at high frequencies to less than optimum values. For example, a recent state of the art monolithic microwave GaAs FET amplifier described by Kim, Tserng, and Shih in "IEEE Electron Device Letters", vol. EDL-7, No. 2, February 1986 achieved 10 dB gain at 44 Ghz.
As system requirements for amplification of signals in excess of 90 GHz currently exist, there is a need in the art for a high frequency amplifier offering adequate power gain without significant parasitic feedback coupling.